Although the design of aluminum based printed circuit boards (PCBs) is no different from that for a traditional FR-4 board, the similarities are limited to the imaging and wet-processing operations. To make the design cost-effective and manufacturable, an additional secondary mechanical operation is necessary. Additional considerations are necessary for the solder mask, legend printing, and mechanical fabrications.

Structural Considerations

PCBs constructed with a metallic support base separated by a thin dielectric from the copper conductors of the circuit are also called Insulated Metal Substrates (IMS). Usually, aluminum is the choice for the carrier material because of its lower costs compared to other metals. The dielectric separating the substrate from the conductors has substantial influence on the total performance and it determines the thermal resistance. Depending on the requirement, the dielectric can comprise layers of filled or unfilled epoxy resins.

Although filled resins offer a 3-10-fold higher thermal conductivity as compared to that from FR-4 material, this depends on the filling material and the quantity used. Additionally, the filling material influences the cycle resistance of the board. One of the major considerations the designers face with IMS PCBs is the coefficient of thermal expansion or CTE. As the board transfers heat to the metal part, it expands at a rate different from that of the dielectric and the copper traces. The differences in CTE between the various constituents of the PCB create stress on the solder joint.

To minimize the effect, copper is preferred to aluminum as the base material, as it has a lower CTE. However, this increases the cost of the PCB. Other design variants include IMS with exposed copper, and aluminum with thin insulation layer.

IMS with exposed copper is an optimized variant. Here, the metal of the copper substrate protrudes partially through the insulation layer, and there is no insulation to impede the transfer of heat. Therefore, electronic components can operate at higher currents or higher power levels. However, this requires the connected thermal contact points to be electrically neutral.

Aluminum with a thin insulation layer is a modified form of IMS technology, where a thin layer of ceramic or aluminum oxide is used, rather than the dielectric made of epoxy resin. Depending on the application requirement of dielectric strength, the layer thickness may vary from less than 40 µm to 125 µm. This helps to achieve thermal conductivities of approximately 2 W/mk.

Design Considerations

Considering reliability, experts recommend housing the control part on a separate standard PCB rather than placing it together with the power part on the aluminum based PCB. One of the most important criteria for aluminum PCBs is the minimum distance for drill holes, as the base substrate is a conducting metal.

For double-sided aluminum core circuits, this requires insulating the aluminum core against through-plating. Usually, the aluminum core must be pre-drilled, and excess resin used when press-molding the aluminum core with prepregs.

This opens up the possibilities of manufacturing a multilayer PCB with an aluminum core. In addition, it is also possible to produce multilayer rigid-flex PCBs, by using an aluminum core of 0.5 mm thickness.

Conclusion

Special design rules may apply for your aluminum PCB dependent on the purpose of the PCB and the outcome you are wanting to achieve with your specifications. Standard aluminum PCB’s can be ordered online on our website http://www.pcbglobal.com/quote/aluminium-pcbs/or for any requirements outside the online capabilities, please email your design file to sales@pcbglobal.com

If you are looking for small volume electronic products that must be highly reliable while operating at high frequencies and high insulation in environments with high pressure, high temperature, and high pressure, Metal Core PCBs (MCPCBs) may be a good choice. However, there is other alternative — a ceramic PCB.

Characteristics

A brief overview of the basic structure of ceramic PCBs offers an insight into why they offer such excellent performance. Usually, ceramic PCBs are made from 96-98% Alumina (Al2O3), Aluminum Nitride (AIN), or Beryllium Oxide (BeO). Although for thin or thick film technology, silver palladium (AgPd) is preferred as the conductor material. For the requirement of direct copper bonding, copper is used. Ceramic PCBs can operate in the temperature range of -55°C to +850°C, and they have excellent thermal conductivity ranging from 24-250 W/m-K, depending on whether the ceramic material is Alumina, Aluminum Nitride, or Beryllium Oxide. Ceramic materials exhibit great compression strengths of above 7000 N/cm2, with breakdown voltages of up to 28 KV/mm for 1.0mm thickness. The thermal expansion coefficient under operating temperatures of 50-200°C is about 7.4 ppm/K.

Types of Ceramic PCBs

Depending on the manufacturing method, three basic types of ceramic boards are available in the market:

• Thick Film Ceramic Boards
• Thin Film Ceramic Boards
• DCB Ceramic Boards

Thick Film Ceramic Boards

These are so called because of the thickness of their conductor layer, which may exceed 10 microns, but not more than 13 microns. The conductor layer is usually silver or gold palladium, and printed on the ceramic substrate.

The advantage of thick films on ceramic boards is manufacturers can put interchangeable conductors, semi-conductors, conductors, electric capacitors, or resistors on the ceramic board. After completing the steps of printing and high-temperature sintering, all the components on the board can be laser-trimmed to their desired values.

Thin Film Ceramic Boards

The thickness of the conductor layer in thin film ceramic boards is less than 10 microns and deposited on the ceramic substrate using thin film manufacturing technologies such as electroplating, sputtering, or evaporation. The thin films are useful in producing on-board passive networks, assemblies for micro-components, and hybrid integration of circuits formed by packaging.

Depending on the concentration of component parameters and the distribution of the passive networks, thin film ceramic PCBs may be further categorized into lumped or distributed parameters. While lumped parameters cater to frequencies lower than that used for microwaves, the distributed parameters are meant for operating within the microwave band alone. Usually, the equipment used for manufacturing thin film ceramic boards is more expensive than those used for making thick film types. In addition, the cost of production is higher for thin film technology.

Thin film ceramic PCBs are very useful for analog circuits such as for microwave circuits, as they need to exhibit high accuracy, greater stability, and excellent performance.

DCB Ceramic Boards

Direct copper bonded (DCB) technology represents a special process where a copper foil is bonded on to the ceramic core (AIN or AL2O3) on one or both sides. The bonding takes place under high temperature and pressure.

This type of bonding not only gives the super-thin DCB substrate high bonding strength, but it also has excellent isolation, high thermal conductivity, and fine solderability. Showing high current loading capacity, the DCB ceramic board can be etched similar to normal FR4 PCBs are.

Conclusion

At PCB Global, we have the capability not to only provide ceramic PCB’s, but to also assist you with any design specifications or inquiries you may have regarding the general use and outcome of the purpose of your ceramic board and also determining if a ceramic PCB is the right choice for your requirements. For any questions or if you would like to arrange a quote for your ceramic PCB, please don’t hesitate to contact us as sales@pcbglobal.com

PTFE or polytetraflouroethylene, commercial name Teflon, has an inert molecular structure that makes it an excellent material for use as non-stick coating. PCB fabricators are increasingly using PTFE laminates compared to conventional FR4 materials, because of the unique properties of PTFE that allow it to be used for high frequency applications. Although fabricating PCBs made of PTFE is very similar to those followed for conventional PCBs, fabricators need to be careful in handling the rather soft material, and fine-tune their processes with special emphasis on those areas where PTFE differs from traditional materials because of its unique properties and chemistry.

For instance, PTFE laminates are soft and the surface is able to bend, wrinkle, or dent more readily than their FR4 counterparts. While such surface imperfections are acceptable in consumer electronic circuits, they can significantly affect functional performance in high frequencies. Therefore, PTFE laminates need a flat support while storing, to prevent them from sagging or drooping, as this can set over time.

Surface Preparation for Metallization, Marking, and Multilayering

It is not advisable to prepare the copper surface of a PTFE laminatemechanically. Equipment such as bristles, pumice scrubbers and composite brushes suitable for conventional rigid material should not be used as the soft PTFE substrate could stretch to absorb the stresses leading to unpredictable dimensional results.

To prepare the PTFE surface, the standard process that the PCB industry uses is Sodium Etchants or Plasma Gas Cycling. These processes strips or removes the fluorine from the PTFE surface making it suitable for metallization, marking, and multilayering.

To avoid problems associated with registration resulting from dimensional stretching, fabricators use soap or a degreasing bath to remove the potential organics. They also use chemical cleaning to remove anti-tarnish coating on the copper foil. This typically removes about 40 millionths of an inch from the surface of the foil to promote photoresist adhesion.

Lamination

PTFE and copper films can bond without the use of bonding films and/or prepregs. Usually, fabricators use temperatures of 700F and pressure of 450-500 psi as a starting point for the lamination process. The temperature and pressure changes with ceramic filling and other compositions of the PTFE laminate.

Fabricators also use bonding films with lower melting point for reducing the processing temperatures to about 250-425F. Others may use ceramic filled bonding plies as woven glass reinforced prepreg, requiring process temperatures of 550F.

Drilling

Although there are no hard and fast rules while drilling copper laminated PTFE substrates, it is essential to employ new tools at all times. Typically, slow infeed and higher chiploads are preferred for eliminating spurious laminate fibers or PTFE tailing.

Fabricators achieve additional benefits such as easier drilling and cleaner holes with ceramic-filled laminates, as this material has a modified dielectric constant, and lower CTE. However, ceramics filling increases drill wear by 25-50%.

Metallization and Copper Plating

As pure PTFE laminates have a very high Z-axis CTE, it is necessary to use high tensile plated copper on the walls of through holes. Copper of high ductility reduces the chances of pad lift, barrel cracks, and blistering, as PTFE has an inherently low modulus.

Soldermask

Fabricators use a standard PTFE plasma cycle process prior to application of soldermask to enhance the SMOBC adhesion to the copper. For best results, application of soldermask should preferably be completed within 12 hours of circuit etching.

Conclusion

At PCB Global, we currently fabricate Teflon PCB’s for microwave applications and the defence industry and have the knowledge, experience and capability to advise our customers on the use of Teflon base material PCB’s and how this can be implemented for their custom PCB design for their intended application. For more information or any inquiries of Teflon PCB’s, please proceed to contact us or simply email your Teflon base design file for a fast quotation to sales@pcbglobal.com

Introduction

Although Light Emitting Diodes (LEDs) operate at very high efficiencies, they do produce heat as a byproduct and this has to be removed if the LED is to operate continuously. As the heat generation in a semi-conductor, such as an LED, happens in the PN junction, the only way to remove it is via one of its leads. Manufacturers prepare special Printed Circuit Boards (PCB’s), which help in removing this heat by conducting it away from the lead and junction of the LED mounted on the PCB. IN this case, PCB’s are manufactured using a base of aluminum, and their names vary from aluminum PCB’s, insulated metal substrate (IMS), aluminum clad, metal clad (MCPCB), and thermally conductive PCB’s.

Mostly used as single-sided, aluminum based copper clad PCB’s have a copper foil bonded onto a thin thermally conductive but electrically insulating dielectric, which in turn is bonded onto a thick aluminum base. The copper layer is processed in the regular way to form the traces, while the profile is machined to the necessary size and shape. During the manufacturing process, the aluminum substrate needs protection from the etching chemicals.

Although this arrangement is specifically suited for population with surface mount devices (SMDs), through hole components are also used, in which case, the PCB’s are usually double-sided or hybrid types. Apart from LEDs and power converters, automotive and RF companies also take advantage of the thermally conducting properties of aluminum PCB’s in their applications.

Benefits of Aluminum PCB’s

• Dramatic increase in heat dissipation, as compared to conventional PCB’s using FR-4 material.
• The base of laminate is mechanically rigid.
• High reliability and MTBF, as thermal stresses on components are low.
• Improved heat transfer allows thinner tracks of lower width for high current designs.
• Smaller PCB’s on account of better thermal management, leading to higher component densities.
• Relatively lower cost compared to FR-4 PCB with heat sinks.

Types of Aluminum PCB’s

The single-sided aluminum based copper clad PCB is the most commonly used. However, other configurations are also available. Therefore, one can have hybrid aluminum PCB’s, flexible aluminum PCB’s, multilayer aluminum PCB’s, and Aluminum PCB’s for through-hole components.

Hybrid Aluminum PCB’s

These are usually conventional FR-4 or PTFE grade PCB’s of 2 or 4 layer construction, bonded on to an aluminum base with thermal materials. This assembly improves heat dissipation and rigidity, while acting as a shield.

Flexible Aluminum PCB’s

These are relatively new developments using IMS with flexible dielectrics. The dielectric material is mostly a polyimide resin system with ceramic fillers to improve the electrical insulation, flexibility, and thermal conductivity. This is applied to a flexible aluminum material that allows forming to different shapes and angles. However, this arrangement does not flex regularly.

Multilayer Aluminum PCB’s

High performance power supply designs commonly use this type of PCB’s. They are made of multiple layers of thermally conductive dielectrics combining layers of circuitry within them. Blind vias carry heat to the aluminum layer for dissipation, although the efficiency of heat transfer is not as good as that of single-layer boards.

Through-Hole Aluminum PCB’s

The Aluminum layer forms the core of the multilayer thermal construction. The aluminum layer is pre-drilled and back filled with dielectric. This is done before laminating it with thermal bonding materials on both sides. The completed assembly is then drilled through and plated. Clearances in the aluminum layer maintain electrical insulation.

Conclusion

At PCB Global, we commonly produce aluminum PCB’s and have the knowledge, experience and capability to advise our customers on the use of aluminum PCB’s and how this can be implemented in their specific design, for their intended application. For more information or any inquiries of aluminum PCB’s outside of our online capabilities, please proceed to contact us at sales@pcbglobal.com

Introduction

While designing multi-layer Printed Circuit Boards (PCB’s), one of the most basic elements that the engineer must include is the requirement for interconnected traces/planes on one layer to traces/planes on another. The most efficient technique of achieving this is to use vias. These are small holes drilled into the layers of the PCB, and fitted with a copper tube connecting to pads on either end. The pads in turn connect to the required traces on respective layers.

Why use Vias?

With increasing use of high-density boards, and engineers must reduce trace widths and spacing to accommodate for applications. Vias are another technique of achieving higher density boards by making them in multiple layers. In turn, the design of vias has also been evolving, with designers and engineers trying out different types of vias such as ‘landless’ and ‘swing types’. One of the very effective methods of achieving increased layer density is by using ‘via-in-pad’ designs.

Example

Consider the plight of an engineer in the process of breaking out the connections from an FPGA or BGA package of, for example, 1760 pins with a 1mm pin-pitch. According to the application data sheet of such a package, 6 signal layers are necessary to breakout the connections from all the pins. However, with advanced via techniques, engineers can now accomplish this with only 2 signal layers, resulting in better interconnection as well as being a substantial cost saving option.

Fabricators using the high-density interconnect (HDI) techniques use advanced technology such as buried vias, blind vias, via-in-pad, and micro-via techniques to improve the density of their boards spectacularly. Micro-via techniques involve using lasers to drill holes of very small diameter. Together with the via technologies above, the use of micro-vias results in 24% increased routing density per layer over conventional design processes.

How do they work?

As the name suggests, via-in-pad is a via deliberately placed within the area of a solderable pad. Normally, conventional design practices prevent the use of a via very close to or within a solder pad. Most manufacturers also recommend not using a via this way. The main reason being the via often acts as a wick does during the reflow process, allowing all the solder paste to melt and flow into its hole, leaving the solder pad starved of solder and resulting in an unsoldered joint. This problem is solved by filling and capping the hole of the via-in-pad.

Therefore, just as with any other tool, via-in-pad technology can lead to spectacular results if used properly, or to disastrous consequences if misused. For instance, inadvertently leaving a via-in-pad uncapped under a BGA solder ball can result in the solder paste flowing down into the hole of the micro-via, leaving the joint open. Therefore, it is essential to have every via-in-pad filled and plated over. To be on the safe side, all vias on the board are filled and plated over, and this effectively takes the via out of consideration.

While filling and capping the via-in-hole does solve a major problem, it creates another one—lack of coplanarity. Unless care has been taken to achieve good planarity, there can be a tiny bump or an indent over the via. This can lead to a less reliable assembly, especially with chip scale and BGA packages.

Conclusion

To discuss whether integrating the use of via-in-pad technologies for your PCB specifications, please feel free to contact the team at PCB Global for the most efficient advice followed by a rapid quote.  Please email you design file to sales@pcbglobal.comfor a rapid and competitive quote.